Method of forming sheet or plate material



Sept. 22, 1970 R, D, B TLER ET AL 3,529,457

mmuon OF FORMING SHEET on PLATE MATERIAL Filed Dec. 15, 1967 /17 39 9 g fii? u I b 4-1 0 n o I 4 United States Patent Office 3,529,457 Patented Sept. 22, 1970 3,529,457 METHOD OF FORMING SHEET OR PLATE MATERIAL Roger David Butler, Kidlington, Ian Frederick Bowers, Freeland, and Cedric Charles Edward Colley, Wheatley, England, assignors to Pressed Steel Fisher Limited, Cowley, Oxford, England, a British company Filed Dec. 15, 1967, Ser. No. 690,816 Claims priority, application Great Britain, Dec. 23, 1966 57,753/66; Mar. 8, 1967, 10,802/67; Mar. 15, 1967,

Int. Cl. B21d 26/04 US. CI. 72-56 4 Claims ABSTRACT OF THE DISCLOSURE A method of forming sheet or'plate material that exhibits superplasticity within a limited range of temperatures, wherein a heated liquid is used to heat the material to the required temperature and the liquid is then used to force the sheet material against a forming die; advantageously the sheet is pre-stretched and the fluid is subjected to high frequency vibrations during forming.

This invention relates to a method of forming, or shaping, sheet or plate material that exhibits a superplasticity within a limited range of temperatures.

It is known that within a limited range of temperatures certain metal alloys, such as the eutectoid alloy of aluminium and zinc, and the eutectic alloy of aluminium and silicon, may be in a state such that they have especially low values of resistance to deformation and extremely high plasticity as compared with other similar alloys having different proportions of the elements, or to the same alloy in a different state or at a temperature outside said range. This phenomenon occurs below a certain strain rate at any particular temperature within the said temperature range and is known as superplasticity, and for convenience metal alloys which are capable of exhibiting this phenomenon are termed superplastic in this specification.

The object of this invention is to provide a method of forming or shaping superplastic sheet or plate material.

According to the invention a method of forming or shaping superplastic sheet or plate material includes subjecting one face of the material to a liquid that has been heated to within, or above, that range of temperatures at which the material becomes superplastic until the material has been heated to within the said range of temperatures, and then applying sufficient pressure to the liquid to so force the sheet or plate towards a die that the sheet or plate is thereby formed at a strain rate below the critical strain rate to a shape corresponding to the shape of the die surface.

The liquid is preferably a molten metal such as a low melting point metal alloy, in order to provide rapid heating of the workpiece; but alternatively suitable temperature resisting oils or suitable salts may be used.

The sheet or plate material may be at least partially pre-stretched before it is formed to the required final shape by the method of the invention thereby allowing the wall thickness to be controlled or preventing excessive thinning during final forming. Thus the material may be prestretched as a separate operation by, for example, a vacuum or press forming operation. Alternatively the material may be pre-stretched after it has been heated to within the superplastic range of temperatures by applying the appropriate pressure to the hot liquid with the die in a retracted, inoperative, position, such that the material is forced to assume a bubble shape, and the die can then be moved to its operative position and the pre-stretched material formed to the required shape, or the material may be pre-stretched by forcing a male die against the hot material.

It has been found that the formability of the material may be improved by subjecting it, during forming, to high frequency vibrations above 10 kc./sec.

The vibrations may be applied to the heated liquid; and advantageously the vibrations are such that they produce an air bearing between the superplastic material and the surface of the die tending to prevent mutual contact.

An exemplary embodiment of the invention will now be described, solely by way of illustration with reference to the accompanying drawing which is a schematic, partially-sectioned, elevation of apparatus for effecting the invention.

The forming apparatus shown in the drawing includes a restraining frame 11 comprising a top platen 12 connected by tie posts, such as 13, to a lower platen 14.

A two-way, pneumatic piston and cylinder device 15 having an inlet 16 and an outlet 17, supports a mould casing 18.

Previously cast in the mould casing 18 is a concrete die 19 having a forming surface 20 and incorporating a compressed air inlet duct 21 and electric resistance heating coils 22 the duct 21 being connected to a controlled source of compressed air (not shown) and the coils 22 being controlled by a thermocouple temperature controlling device (not shown) responsive to a temperature adjacent the forming surface 20. The die 19 is fitted with a steel sealing flange 23.

The lower platen 14 supports a concrete back-up block 24 in which is embedded a steel forming chamber 25. Electric resistance heating coils 26 are embedded in the concrete block 24 adjacent the chamber 25 and are controlled by a thermocouple enclosed in a metal or refractory sheath 27 extending into the chamber 25 and con nected to a temperature controlling device (not shown).

A high frequency signal generator 28 has a horn 29 which extends into the chamber 25, and is sealed by a high temperature O-ring 30.

The chamber 25 is connected by a pipe 31 to a reservoir 32. The reservoir 32 is surrounded by electric resistance heating coils 33 controlled by a thermocouple enclosed in a metal or refractory sheath 34 and connected to a temperature controlling device (not shown), and the pipe 31 is surrounded by further electric resistance heating coils 35 of which those adjacent the reservoir 32 are controlled by a thermocouple in the sheath 34 whilst those adjacent the forming chamber 25 are controlled by the thermocouple in the sheath 27.

The top of the reservoir 32 is connected by the pipe 36 and through a pressure regulating valve 37 to a compressed air source inlet 39, and is also connected by the branch pipe 38 through a metering valve 40 to a variable preset metering pump 41. Furthermore the pipe 36 is provided with a pressure release valve 42.

The reservoir 32, the pipe 31 and the forming chamber 25 are filled with a low melting fusible metal alloy 43 having a melting point of C. and maintained at 260 C. by the resistance heaters 26, 33 and 35; and the forming surface 20 of the die 19 is also maintained at 260 C. by the heating coils 22.

In operation, a superplastic sheet workpiece 44 is placed as shown in the drawings: the workpiece may be formed of the eutectoid aloy of zinc and aluminium which has been prepared by quenching the sheet in water from 325-350 C. to room temperature in order to induce the required superplastic state.

In this example the workpiece 44 has been prestretched by conventionally forming it in a press to the shape shown in the drawing. The workpiece 44 is placed in position as shown and the pneumatic device 15 is operated to move the die 19 towards the forming chamber 25 until the sealing flange 23 is loosely holding the workpiece 44; the release valve 42 is closed, and the preset metering pump 41 is actuated to increase the pressure on the hot alloy 4-3 sufficiently to cause the alloy 43 in the forming chamber 25 to contact the workpiece 44 and thereby raise the temperature of the workpiece to 260 C., the air within the chamber 25 being displaced past the periphery of the workpiece 44.

The device 15 is then operated to firmly grip the workpiece 44 and effect a seal between the sealing flange 23, the workpiece 44 and the fluid chamber 25. The signal generator 28 is energised and high pressure compressed air is admitted through the inlet 39, the pressure regulating valve 37 and the pipe 36 into the reservoir 32, and thereby the hot alloy 43 in the forming chamber 25 forces the workpiece 44 towards the surface 20 of the die until the workpiece conforms to the shape of the surface 20. During forming it will be understood that the air pressure and restrictions to flow must be such that the critical value of strain rate is not exceeded.

After the workpiece 44 has been formed to the required shape, the electric heating coils 22 are turned off, and the pressure release valve 42 is opened to allow the level of the hot alloy 43 in the forming chamber 25 to fall.

The pressure in the pneumatic device 15 is then released.

As claimed in our co-pending application, compressed air at a relatively low pressure is admitted through the duct 21 to effect cooling of the workpiece; and after the workpiece has been cooled to a temperature at which it is sufiiciently rigid, the air pressure is increased to eject the workpiece from the die 19.

The pneumatic device 15 is then operated to lift the die 19, and the workpiece is removed.

What we claim is:

1. A method of forming superplastic metal sheet or plate material including the steps of:

(a) heating a low melting point alloy to at least a temperature at which said alloy is liquid,

(1)) heating said sheet material to a temperature at which it is superplastic to thereby exhibit strain rate sensitivity by bringing said heated liquid alloy into contact with one face of said sheet material,

() forcing said sheet material to conform to the surface of a die by exerting sufficient pressure on said heated liquid alloy to deform the said sheet material while subjecting said liquid alloy and hence said sheet material to high frequency vibrations.

2. Apparatus for forming a strain rate sensitive, a superplastic metal alloy workpiece, which comprises in combination:

. (a) a chamber having an open end,

(b) a low melting point metal alloy in said chamber,

(c) means for heating and thereby melting said low melting point metal alloy in said chamber,

(d) means for introducing high frequency vibrations in said low melting point metal alloy in said chamber,

(e) clamping means for firmly holding the periphery of a workpiece against the open end of said chamber,

(f) means for moving said molten metal alloy within said chamber into contact with a workpiece clamped against the open end of said chamber,

(g) means for increasing the pressure on said alloy as it deforms a workpiece clamped against the open end of the said chamber.

3. The apparatus of claim 2 wherein said (g) means comprises a controlled source of compressed gas.

4. The method of claim 1 wherein said (0) high frequency vibrations are at least 10 kHz.

References Cited UNITED STATES PATENTS 1,793,054 2/1931 Carnes 7260 2,348,921 5/ 1944 Pavlecka 7260 2,382,045 8/1945 Flowers 7256 3,172,928 3/1965 Johnson 7256 3,201,967 8/1965 Balamuth et al. 7256 2,393,131 1/1946 Vang 7256 2,728,317 12/1955 Clevenger et al. 7260 2,770,874 11/1956 Lindow 29-421 3,340,101 9/1967 Fields et al. 7260 OTHER REFERENCES Superplasticity in an Al-Zn Alloy by Backofen et al., pp. 980990 of Transactions of ASM, vol. 57, 1964.

RICHARD J. HERBST, Primary Examiner US. Cl. X.R. 

